<!doctype html>
<script>
function handler(event) {
    window.top.postMessage(event.sourceCapabilities.customProperty, "*");
}

function runTest() {
    document.querySelector("button").click();
}

window.onload = function () {
    setTimeout(runTest, 1000);
    window.top.postMessage("start", "*");
}
</script>
<button onclick="handler(event)"></button>
